The cytoplasmic site of gene expression and use of virally encoded enzymes is a distinguishing feature of vaccinia virus and other poxvirus vector systems that probably accounts for their consistent ability to express foreign genes derived from a variety of prokaryotic, eukaryotic, and viral sources. This feature, together with their ability to stably integrate and package large amounts of DNA without loss of infectivity, their wide host range, and the development of simple and effective methods for isolating recombinant viruses account for their diverse use and popularity. Increased expression of genes cloned into the vaccinia virus genome has been achieved primarily through the use of mutated and synthetic promoters. The current levels of expression are up to 50-fold higher than that achieved with the original vectors. An alternative procedure, in which the RNA polymerase and regulatory sequences of a bacteriophage are used, also has been developed. Initially, the transcriptional components of bacteriophage T7 were used and this has now been extended to SP6 thereby increasing the versatility of the system. Several approaches have been used to develop highly attenuated vaccinia virus vector strains that would be safer in the laboratory and more suitable as live vaccines. The first has been to identify and delete genes that are not required for replication in tissue culture but which contribute to virulence in animals. The effects of deletion of the genes encoding the vaccinia virus complement control protein, which would allow more rapid clearance of the virus by the host and the VP37 envelope protein, which would limit cell-to-cell spread of virus, are currently being tested. Another approach is to evaluate strains of vaccinia virus that were attenuated by extensive tissue culture passage during the era of small pox vaccination. We determined that the highly attenuated MVA strain, which cannot produce infectious virus in mammalian cells, still has the capacity to express foreign genes at a very high level.